The present invention relates to an optical information recording/reproducing device for reading information from and/or writing information to multiple types of optical discs (differing in data density, protective layer thickness, etc.), an optical element installed in the optical information recording/reproducing device, and a design method of the optical element.
There exist various standards of optical discs (CD, DVD, etc.) differing in data density, protective layer thickness, etc. Meanwhile, new-standard optical discs (HD DVD (High-Definition DVD), BD (Blu-ray Disc), etc.), having still higher data density than DVD, are being brought into practical use in recent years to realize still higher information storage capacity. The protective layer thickness of such a new-standard optical disc is substantially equal to or less than that of DVD. In consideration of user convenience with such optical discs according to multiple standards, the optical information recording/reproducing devices (more specifically, objective optical systems installed in the devices) of recent years are required to have compatibility with at least two (preferably, three) of the above optical disc standards. Incidentally, in this specification, the “optical information recording/reproducing devices” include devices for both information reading and information writing, devices exclusively for information reading, and devices exclusively for information writing. The above “compatibility” means that the optical information recording/reproducing device ensures the information reading and/or information writing with no need of component replacement even when the optical disc being used is switched.
In order to provide an optical information recording/reproducing device with the compatibility with optical discs of multiple standards, the device is generally configured to be capable of forming a beam spot suitable for the particular data density of the new disc (in the switching of the optical disc to the new disc of a different standard) by changing the NA (Numerical Aperture) of the objective lens employed for the information read/write, while also correcting spherical aberration which varies depending on the protective layer thickness. Since the diameter of the beam spot generally gets smaller as the wavelength of the beam gets shorter, multiple laser beams having different wavelengths are selectively used by the optical information recording/reproducing device depending on the data density of the disc. For example, for DVDs, a laser beam with a wavelength of approximately 660 nm (shorter than approximately 790 nm for CDs) is used. For the aforementioned new-standard optical discs, a laser beam with a wavelength still shorter than that for DVDs (e.g. so-called “blue laser” around 408 nm) is used in order to deal with the extra-high data density.
Meanwhile, in order to finely converge each light beam (being used) exactly at the read/write position on the record surface of the corresponding optical disc, a well-known method employs an “annular zone structure” (including a plurality of concentric annular zones with a minute step between adjacent zones) on one or more arbitrary-selected surfaces of at least one optical element (e.g. objective lens) of the objective optical system. By the effect of the annular zone structure, light beams of different wavelengths can be finely converged on the record surfaces of the corresponding optical discs, respectively.
Optical pickup devices having the aforementioned compatibility with two types of optical discs having different data densities have been proposed in Japanese Patent Provisional Publication No. 2005-203043 (hereinafter referred to as JP 2005-203043A) and Yoshiaki Komma et al. “Compatible objective lens for blu-ray disc and DVD using diffractive optical element and phase-step element which corrects both chromatic and spherical aberration (ISOM 2003 proceedings (We-F-20), Matsushita Electric Co., Ltd. (hereinafter referred to as a “document #1”), for example.
Meanwhile, an optical pickup device having the compatibility with three types of optical discs having different data densities has been proposed in Japanese Patent Provisional Publication No. 2001-195769 (hereinafter referred to as JP 2001-195769A), for example.
The optical pickup device disclosed in the JP 2005-203043A corrects relative spherical aberration which is caused by the difference between the protective layer thicknesses of the two types of optical discs, by employing an optical element having the annular zone structure (including a plurality of concentric annular zones with a minute step between adjacent zones) on one surface.
Meanwhile, in the document #1, two lens surfaces in the objective optical system are provided with different diffracting structures having different effects, by which each of the laser beams corresponding to DVD and BD can be finely converged on the record surface of the corresponding optical disc (DVD, BD) while also correcting a change in the spherical aberration caused by wavelength variation.
In JP 2001-195769A, a diffracting structure is formed on both surfaces of the objective lens, by which each of the laser beams corresponding to HD DVD, DVD and CD can be finely converged on the record surface of the corresponding optical disc (HD DVD, DVD, CD).
However, forming the diffracting structure on two lens surfaces as above tends to result in performance degradation deriving from manufacturing errors (decentering, etc.). Further, when the objective lens optical system is composed of a single lens as in the optical pickup device of JP 2001-195769A for reducing the number of components and the cost, the diffracting structure on one lens surface is exposed to the outside via the disc tray and the exposed diffracting structure has a high possibility of being destroyed when a lens cleaner is used, for example.